Happening plugin: challenge your friends for a game of chess.
In case you don't know what this is about: Happening is a group app for iOS and Android, that allows you to create your own plugins in minutes. To get started signup at: https://develop.happening.im
Don't worry: plugins are just Javascript (or CoffeeScript), both client-side and server-side. However, the powerful Happening environment provides you with user group details, a database that syncs to all clients, reactive user-interface widgets, and html/css for whatever else you may want to do. That social app you've wanted to build for ages, but didn't because it'd take you weeks? You'll finish it tonight.